When a subsea intervention stack is used for intervention work in subsea wells, producers or injectors, the subsea intervention stack has to be flushed both prior to, and after each wireline run. This is needed to flush seawater out of the subsea lubricator to prevent seawater from entering the wellbore but most importantly, to prevent hydrates forming when hydrocarbon comes into contact with free water. A hydrate inhibiting fluid, for example monoethylene glycol (MEG), is normally used.
Most commonly today MEG is supplied from a surface vessel by means of a hose or umbilical to the subsea intervention stack during an intervention operation. In todays systems, more MEG than needed is usually supplied to the subsea stack to be certain that no hydrates will form. One disadvantage is the larger costs involved with the MEG consumption, another is the environmental aspect in the cases where MEG is flushed to sea.
In addition to the mentioned issues, the use of hoses from surface is considered as costly and unwanted in deep water intervention work
In WO 01/25593 (belonging to the applicant) it was suggested to use a flushing system that enabled the MEG and hydrocarbons in the stack bores to be flushed into the well or into the flowline. This avoided discharge to sea or bringing hydrocarbons to the surface, but had the disadvantage that forcing MEG of high pressure into the well might disturb the formation. Another disadvantage is that this system is also dependent upon hoses or umbilicals to supply the needed MEG to the subsea stack.
Moreover, U.S. Pat. No. 3,500,907 describes a closed flushing and vapour elimination system for cleaning wireline tools under conditions such as a subsea chamber at an underwater wellhead, where toxic and unpleasant fumes could be harmful to human operators. The system comprises two fluid lines for connection to a lubricator assembly, a pump and a fluid container.
The object of the present invention is to provide a method and device for circulating fluid in a subsea module where the above disadvantages are avoided.
It is an object of the invention to reduce the consumption of MEG, especially the volume of MEG that is dropped into the well. Moreover, it is an object to enable for cost efficient subsea intervention in deep water with use of a subsea intervention stack system.
Moreover, it is an object of the invention to allow for low power consuming subsea pump technology to handle the circulation of a fluid in a subsea intervention stack.